Both actually!
My mother's mother's side of the family surname is Lewis. Since there was only girls on that side, the surname ended with my Great Grandfather - so, as a show of respect and to keep the name going, it became a christian name to all boys on my grandmother's side - my Uncle's christian name was Lewis, as am I, irrespective of surname. If I have any boys (not likely), they would be Lewis too, but considering I don't, and there are no plans for more kids, I will ask my girls to call their boys Lewis.
There ya go
